Various proposals have heretofore been made on capsules composed of a core material and a capsule shell covering the core material. Among these proposals, microcapsules whose capsule shell have been partially or entirely formed by interfacial polymerization or in-situ polymerization are excellent in the completeness of covering of the core material and the retention of inner materials. Such microcapsules have been put into practical use, e.g., non-carbon paper and pressure measuring paper.
Since the capsule shell obtained by interfacial polymerization or in-situ polymerization nominally has a thickness of 0.5 .mu.m or less, it is necessary that multifunctional capsule shell-forming monomers be used to form a polymer having a crosslinked structure so as to retain a core material, especially a core material containing a volatile liquid for a long period of time. However, the capsule shell made of a polymer having a crosslinked structure is disadvantageous in that it is brittle and thus has a poor mechanical strength. In particular, in the case where microcapsule grains are used in the form of powder, this disadvantage causes a great problem. Furthermore, since the capsule shell obtained by interfacial polymerization or in-situ polymerization is limited to polycondensation polymers or polyaddition polymers such as polyurea, polyurethane, polyamide, polyester and epoxy resins, it is difficult to obtain microcapsules provided with a special function or to modify the surface of microcapsules so as to provide the microcapsules with such a special function.
JP-A-62-227161 and JP-A-62-227162 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") propose an approach which comprises the use of graft polymerization to provide capsule shell with a special function. However, polycondensation polymers or polyaddition polymers such as polyurea, polyurethane, polyamide, polyester and epoxy resins are disadvantageous in that they have a small capability of initiating graft polymerization. Therefore, a two-stage processing must be carried out in which monomers having a plurality of vinyl groups per molecule are bonded to a capsule shell with a catalyst such as a cerium salt, and monomers having a special function are then polymerized to the above vinyl group as an initiation point. Further, since this process essentially produces free polymers which are not grafted, reducing the efficiency of graft polymerization to the capsule shell, the improvement in the retention of the inner materials cannot be improved.